An internal combustion engine emits exhaust gases consisting of products from the combustion of fuel within the engine. During complete combustion, the hydrocarbons (HC) in the fuel are broken down to release energy and form carbon dioxide (CO.sub.2) and water (H.sub.2 O). Complete combustion requires a chemically balanced, or stoichiometric reaction. Unfortunately, neither optimum power generation nor optimum fuel economy occurs under these conditions. Furthermore, it is very difficult to maintain the proper conditions for complete combustion due to the large number of variables present in an operating engine.
Products of incomplete combustion of an air fuel mixture include carbon monoxide (CO), oxides of nitrogen (NO.sub.x), and hydrocarbons. Increasingly stringent federal regulations limit the permissible levels for emissions of these gases. As such, vehicle manufacturers have developed various methods to reduce emissions while improving vehicle performance and fuel economy.
Exhaust emissions of regulated gases may be reduced by closely controlling the mass ratio of air to fuel provided to the engine. For a gasoline powered internal combustion engine, the mass ratio for complete fuel combustion is about 14.7:1, i.e. 14.7 kilograms of air to 1 kilogram of fuel. This ratio is known as the stoichiometric ratio.
The air fuel ratio is often described in terms of the excess-air factor, lambda (.lambda.). This ratio indicates the deviation of the air fuel ratio from the theoretically required stoichiometric ratio. At the stoichiometric ratio, lambda is equal to 1. For a mixture with excess air, also known as a lean mixture, lambda is greater than 1. For a mixture with deficient air or excess fuel, also known as a rich mixture, lambda is less than 1. For rich mixtures, HC and CO emissions increase while No.sub.x emissions decrease. The converse is true for lean mixtures. As such, to minimize emissions, it is desirable to operate the engine at the stoichiometric ratio.
Closed-loop control is effected by the Engine Control Module (ECM) to regulate the air fuel ratio under various operating conditions. Because it is difficult to measure many of the variables which affect the combustion process, the ECM controls lambda using an exhaust gas oxygen sensor coupled to the engine exhaust stream. This sensor provides a feedback signal to the ECM indicative of oxygen content of the exhaust gas.
One such sensor, well known in the art, is the heated exhaust gas oxygen (HEGO) sensor. The HEGO sensor is generally a bistable device which provides a voltage of about 0.1 volts when oxygen is detected and about 0.8 volts when oxygen is not detected, generally corresponding to lean and rich mixtures, respectively. The ECM then uses the sensor signal to modify the air fuel mixture supplied to the engine to maintain the stoichiometric ratio.
To further reduce emissions, a catalytic converter is typically installed in the exhaust system of the engine. The converter includes a catalyst to promote chemical reactions that transform the exhaust emissions to primarily water and carbon dioxide. The most commonly used converter is the three-way converter (TWC). As the name implies, it simultaneously reduces the concentration of all three regulated exhaust gases: HC, CO, and NO.sub.x.
The catalyst includes catalytic sites which have the ability to store and release oxygen to promote reactions that oxidize HC and CO, converting them into CO.sub.2 and H.sub.2 O, while reducing NO.sub.x into N.sub.2. The catalytic sites may become contaminated and lose their ability to store and release oxygen as the converter ages. The filtering effect, or conversion efficiency, of the catalyst decreases as the number of contaminated sites increases. The efficiency is a measure of the effectiveness of the catalyst in transforming the exhaust gases to water and carbon dioxide.
To monitor the conversion efficiency of the catalytic converter, an additional exhaust gas oxygen sensor may be used. Due to its function, this sensor may be referred to as the catalyst monitoring sensor (CMS) even though it functions in a similar manner as the HEGO sensor. While the HEGO sensor is positioned upstream of the converter, between the engine and the converter, the CMS is positioned downstream of the converter.
One method known in the art for monitoring converter efficiency is to calculate a ratio of CMS transitions or switches to HEGO transitions or switches. If the ratio is high, the catalyst has a small filtering effect and its efficiency is low. If the ratio is low, the catalyst has a large filtering effect and its efficiency is high. Thus, the bulk conversion efficiency of the catalyst may be inferred from the switch ratio, which generally increases over time.
Catalytic converter efficiency may vary greatly depending upon the particular operating conditions. For example, catalyst temperature, exhaust gas mass flow rate, and exhaust gas concentration may affect the ability of the catalyst to operate properly. Thus, to detect bulk degradation of the catalyst, it is necessary to monitor the conversion efficiency over time under similar operating conditions. An accurate determination of the converter efficiency is necessary to reduce unnecessary warranty repairs or the potential of a poor catalyst being undetected.
While the prior art methods are sufficient to monitor conversion efficiency to meet current emission regulation levels, they may be less accurate under the progressively stricter regulations for low emission vehicles (LEVs) and ultra-low emission vehicles (ULEVs). As such, it is desirable to improve the sensitivity of the catalytic converter monitor while increasing its repeatability and confidence level to reduce unnecessary warranty costs.